Due to the worldwide growing energy crisis, energy conservation and emission reduction are more important today than ever before. In December of 2009, United Nations Environment Programme reported that building energy consumption occupies about one third of global greenhouse gas emissions. In China, building energy consumption overall accounts for 30% of the total available primary energy. In particular, energy exchange through windows accounts for over 50% of energy consumed through a building's envelope by means of conduction, convection and radiation. To reduce energy consumption, it is necessary to develop smart windows which are designed to intelligently control the amount of transmitted light and heat (mainly in the near infrared region) in response to an external stimulus.
At present low emissivity glass which has high visible transmittance and high infrared reflection is prevalent in the energy saving glass market and can greatly reduce heat transfer from indoors to outdoors compared to the ordinary glass and traditional building coating glass. However, low emissivity glass is expensive and not intelligent enough. Therefore, there is urgency to develop the next generation of smart windows with independent intellectual property rights.
Vanadium dioxide (VO2) with a Mott-phase transition is a key material for application to thermochromic smart windows because it exhibits a reversible transformation from an infrared-transparent semiconductive state at low temperatures to an infrared-transparent semiconductive state at high temperatures, while maintaining visible transmittance.
Various techniques including the sol-gel method, chemical vapor deposition, sputtering deposition, pulsed-laser deposition, and ion implantation have been utilized to deposit VO2 films, however many problems exist, such as expensive equipment, complex control processes, poor stability, low deposition rate and unsuitable mass production. In addition, the application of smart windows with VO2 films is restricted because it can only be applied to new glass. Therefore, on the basis of energy saving reconstruction, VO2 powders with intelligent energy-saving effect are preferably coated on existing ordinary glass.
The vanadium-oxygen phase diagram shows nearly 15-20 other stable vanadium oxide phases besides VO2, such as VO, V6O13 and V7O13. The formation of VO2 occurs only over a very narrow range of oxygen partial pressures. Additionally, more than ten kinds of crystalline phases of vanadium dioxide have been reported, including tetragonal rutile-type VO2 (R), monoclinic rutile-type VO2 (M), triclinic VO2 (P*(2)), tetragonal VO2 (A), monoclinic VO2 (B), VO2 (C), orthorhombic VO2.H2O, tetragonal VO2.0.5H2O, monoclinic V2O4 and V2O4.2H2O. Only the rutile-type VO2 (R/M) undergoes a fully reversible metal-semiconductor phase transition (MST) at approximately 68° C. However, the preparation of VO2 (M/R) powder has become a technical difficulty for the application of smart windows.
High temperature sintering was usually used to fabricate VO2 powder. A method to fabricate vanadium dioxide powder doped with tungsten, in which VO2 (B) powder is first synthesized and then heat treated at 350˜800° C. to attain VO2 (R) powder is issued in patent (CN10164900). Moreover, many methods including spray pyrolysis (U.S. Pat. No. 5,427,763), thermal cracking (CN1321067C), sol-gel (U.S. Pat. No. 6,682,596) and reverse microemulsion (WO2008/011198 A2) have been used to synthesize VO2 powder. A patent (CN101391814) from our research group describes one-step hydrothermal synthesis of VO2 (M/R) powders.